A touch sensor may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) within a touch-sensitive area of the touch sensor overlaid on a display screen, for example. In a touch sensitive display application, the touch position sensor may enable a user to interact directly with what is displayed on the screen, rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a desktop computer, laptop computer, tablet computer, personal digital assistant (PDA), smartphone, satellite navigation device, portable media player, portable game console, kiosk computer, point-of-sale device, or other suitable device. A control panel on a household or other appliance may include a touch sensor.
There are a number of different types of touch position sensors, such as (for example) resistive touch screens, surface acoustic wave touch screens, and capacitive touch screens. Herein, reference to a touch sensor may encompass a touch screen, and vice versa, where appropriate. When an object touches or comes within proximity of the surface of the capacitive touch screen, a change in capacitance may occur within the touch screen at the location of the touch or proximity. A touch-sensor controller may process the change in capacitance to determine its position on the touch screen.
Poor coupling between a user of a device and the device itself may lead to inaccurate measurements and cause actual touches to be missed or inaccurately reported. Effects caused by poor coupling between the user and the device may be referred to as free space effects. One such example is retransmission of drive signal from a drive line on which one touch is present to a sense line on which another touch is present (or when the same touch is present on multiple sense lines and/or multiple drive lines, e.g., when the device is touched by a large finger, a palm, or a cheek) when the user is touching two or more nodes simultaneously resulting in an unexpected amount of drive signal coupled to (and measured on) the sense line. The amount may be unexpected when compared to the amount of drive signal coupled to the sense line when only one node is being touched. This may affect the accuracy of touch sensing devices.